This invention relates to a carrier phase recovery subsystem employed in fixed Broadband Wireless Access (BWA) applications operating in adaptive burst modems and multi-link hopping mesh radio networks over slow time-varying channels. The adaptive modem is capable of fast link-hopping from one link to another over such channels. That is the channel is quasi-static from burst to burst for any given link.
The embodiments described herein may be used in conjunction with a wireless mesh topology network of the type described in U.S. patent application Ser. No. 09/187,665, entitled “Broadband Wireless Mesh Topology Networks” and filed Nov. 5, 1998 in the names of J. Berger and I. Aaronson, and with network nodes including switched multi-beam antenna designs similar to the design described in U.S. patent application Ser. No. 09/433,542, entitled “Spatially Switched Router for Wireless Data Packets” and filed in the names of J. Berger, et al., as well as with the method and apparatus disclosed in U.S. patent application Ser. No. 09/699,582 entitled “Join Process Method For Admitting A Node To A Wireless Mesh Network, filed Oct. 30, 2000 in the names of Y. Kagan, et al. Each of these U.S. patent applications is incorporated herein by reference.
Burst transmission of digital data is employed in several applications such as satellite time-division multiple access, digital cellular radio, wideband mobile systems and broadband wireless access systems. The design trade-offs and the resulting architectures are different in each of these applications.
In most of these applications, a preamble of known symbols is inserted in the beginning of each burst of data packets for synchronization purposes. Data-aided (DA) algorithms are normally employed for good performance for short preamble. However, synchronization becomes difficult at low values of signal-to-noise ratio (SNR).
Coherent detection schemes are superior compared to differential coherent or non-coherent schemes in terms of power efficiency. However, carrier phase recovery required for coherent detection is influenced by a time-varying characteristic of a fading channel. The power efficiency presented by coherent detection in a digital communication system is effective only if carrier phase synchronization techniques are provided for the receiver.
A constant need for ever-increasing throughputs through fixed bandwidths, fueled by broadband IP (Internet protocol) applications, has pushed system designers toward more throughput-efficient modulation schemes. Because of their relatively good performance, large quadrature amplitude modulation (QAM) constellations are being used in many of these applications. One of the critical problems associated with the use of large QAM constellations is that of carrier phase estimation, which for efficiency reasons, must often be done without the use of a preamble, particularly in burst modem applications. The problem is further complicated for cross-talk interference between the quadrature components (i.e., I/Q channels).
For coherent detection, there are two basic approaches to establish carrier phase synchronization at the receiver. One is pilot based where a known signal is inserted at the transmitter that allows the receiver to extract the pilot symbol and synchronize its local oscillator to the carrier phase of the received signal. Known symbols are multiplexed with the data sequence in a ratio of p pilots to m data symbols. At the receiver, the incoming waveform is filtered and sampled at the symbol rate. The sample sequence is split into two streams; a data stream and a reference stream of known symbols. The latter is decimated and only the samples corresponding to pilot symbols are used for further processing. A narrow band phase-locked loop (PLL) is typically employed to acquire and track the carrier component of the received signal.
Such an approach is not appropriate in applications involving transmission of short bursts. The insertion of a known data sequence greatly reduces the transmission efficiency for a short burst. As a result, pilot-aided algorithms are not applicable in such systems.
In the second approach, the carrier phase estimate is derived directly from the modulated signal. This approach is much more prevalent in practice due to its distinct advantage that the total transmitter power is allocated to the transmission of the data symbols. Transmission efficiency is optimized.
The effect of carrier phase error, φe=φ−{circumflex over (φ)}, in high-level modulation schemes, such as M-QAM is to reduce the power of the desired signal component by a factor of cos2 (φ−{circumflex over (φ)}) in addition to the cross-talk interference from the in-phase and quadrature components. Since the average power level of the in-phase and quadrature components is the same, a small phase error causes a large degradation in performance, particularly at higher modulation levels (i.e., M≧16).
In continuous modem applications, the user is typically willing to wait a few seconds while the receiver goes through an acquisition phase in which tracking processes converge. Often, the acquisition process in a continuous modem simply allows phase-locked loops to pull in on the received signal. In other words, the acquisition processing is not different from the tracking processing.
In contrast, in a burst modem, the user data content of a given transmission may be only a fraction of a millisecond. Long acquisition times contribute an unacceptable level of overhead to the system and substantially reduce capacity. Thus, the burst modem requires a special acquisition process that will quickly estimate the appropriate receiver gain, the carrier frequency and phase, the sample timing frequency and phase, and, if needed, the equalizer taps for an equalizer of the receiver. Also, the acquisition process must reliably identify which bit in the burst is the first user data bit so that higher layers of the protocol stack can format data properly.
The initial carrier phase can be estimated using the phase of the output of a coherent correlator. This phase is an estimate of the phase of a sample in the middle of the preamble. If we desire estimates of the phase at the beginning or the end of the preamble, the estimate from the middle must be compensated by the frequency error estimate. Since the frequency error estimate is not perfectly accurate, use of it for this compensation will degrade the accuracy of the phase estimate formed for the preamble ends. Thus, it should be carefully considered whether to start tracking from either end of the preamble or whether starting in the middle is the best route.
Another important design decision is the choice of the estimator topology. In continuous modem applications, closed-loop (feedback) structure is commonly used with relatively high performance depending on the application. However in burst modem systems, closed-loop structures do not produce the best results. Feedback systems require, in general, longer tracking time (i.e., long data sequence) for an acceptable performance level in many applications. This requirement is typically not met in burst modems where the burst length could be in the low microseconds (i.e., 20 to 40 octets).
Accordingly, there is a need for a method and apparatus for carrier phase recovery in a burst mode system. Further, there is a need for a method and apparatus for carrier phase recovery in a link hopping system using transmission bursts for radio communication.